Shape memory alloy articles with improved fatigue performance and methods therefore

ABSTRACT

Articles made of shape memory alloys having improved fatigue performance and to methods of treating articles formed from shape memory alloy materials by pre-straining the articles (or desired portions of the articles) in a controlled manner so that the resultant articles exhibit improved fatigue performance. The shape memory articles are preferably medical devices, more preferably implantable medical devices. They are most preferably devices of nitinol shape memory alloy, most particularly that is superelastic at normal body temperature. The pre-straining method of the present invention as performed on such articles includes the controlled introduction of non-recoverable tensile strains greater than about 0.20% at the surface of a desired portion of a shape memory alloy article. Controlled pre-straining operations are performed on the shape-set nitinol metal to achieve non-recoverable tensile strain greater than about 0.20% at or near the surface of selected regions in the nitinol metal article.

CROSS REFERENCE TO RELATED APPLICATIONS

The present application is a continuation of U.S. patent applicationSer. No. 12/873,191 filed Aug. 31, 2012, now U.S. Pat. No. 8,216,396,which is a continuation of U.S. patent application Ser. No. 10/428,872,filed May 2, 2003, now U.S. Pat. No. 7,789,979.

FIELD OF THE INVENTION

The present invention relates to the field of shape memory alloys,medical articles made from shape memory alloys and more specifically toshape memory alloy articles having improved fatigue resistance, andmethods of making such articles.

BACKGROUND OF THE INVENTION

Shape memory alloys have been used for a variety of applications sincethe discovery of shape memory transformation by Chang and Reed in 1932.Nitinol, the near-equiatomic alloy of nickel and titanium (optionallycontaining ternary, quaternary or more elements such as copper,chromium, iron, vanadium, cobalt or niobium) that thus far offers themost useful shape memory and superelastic properties, was discovered byBuehler and his colleagues in 1962.

Nitinol has proven to be adequately biocompatible for a variety ofmedical device applications, including implantable applications. It hasbeen used for orthodontics, in a variety of orthopedic devices, forfilter devices in various body conduits and for stent devices formaintaining patency of various body conduits, particularly those of thevasculature. These stent devices (including stent-grafts, i.e., stentsprovided with a flexible covering of a vascular graft material such asporous expanded polytetrafluoroethylene) are generally inserted into abody conduit at a site remote from the intended implantation location,and transported to the desired location by a catheter or similar device.They are usually inserted in a collapsed or compacted state to enabletheir movement through the body conduit to the desired implantationsite, at which location they are expanded to the desired size tointerferably fit within the conduit and hold the conduit open at thatlocation. While these devices are most often used for cardiacapplications, they are also used for the repair of thoracic andabdominal aortic aneurysms and for peripheral and carotid applications.

Many of these stent devices are made from materials intended to beexpanded by the application of a force applied internal to the tubulardevice, usually by the inflation of a catheter balloon on which thedevice was mounted for insertion into the body conduit. Theseballoon-expandable devices are most often made from a plasticallydeformable material

such as a stainless steel. Many other stents are made from shape memorymaterials, particularly nitinol, and take advantage of the shape memoryor superelastic properties so that they may be implanted simply byreleasing the constrained, compacted device and allowing it toself-expand at the desired implantation site.

Stent devices should be adequately flexible to enable them to bedelivered through bends in the sometimes-tortuous pathways of a bodyconduit. They may also need to be adequately flexible to conform tobends in the body conduit at the implantation site, and to be able toaccommodate movement of the body conduit. This is particularly true inthe vasculature, where a vessel often changes dimension as a function ofsystole and diastole. These devices consequently should also have goodfatigue resistance.

Shape memory materials can exhibit pseudoelastic (superelastic)behavior, allowing the material to recover a significant amount ofstrain due to the reversible, isothermal metallurgical phasetransformations by changes in the state of stress. The superelasticbehavior is characterized by a linear elastic and a nonlinearpseudoelastic stress-strain response allowing the material to recover asignificant amount of strain due to the reversibleaustenitic-martensitic phase transformation. Conventional nitinolmaterials can typically recover principle strains on the order of up to8% (see “Nitinol Medical Device Design Considerations” by Philippe P.Poncet, SMST-2000: Proceedings of the International Conference on ShapeMemory and Superelastic Technologies, pp. 441-455). The superelasticbehavior of nitinol allows for the design of devices that exert arelatively constant stress over a wide range of strains or shapes. Thisunique behavior has been utilized in the design of many implantablemedical devices such as stents and stent-grafts.

The phase stability of nitinol is a function of both temperature andstress. The phase stability in the unstressed state is characterized bythe transformation temperatures M_(f), M_(s), A_(s), and A_(f).Martensite is the stable phase at temperatures below M_(f), themartensitic finish temperature. Upon heating, the martensitic structurebegins a reversible thermoelastic phase transformation to austenite whenthe temperature reaches A_(s), the austenitic start temperature. Thetransformation to austenite is completed when the temperature reachesA_(f), the austenitic finish temperature. Upon cooling the austenite,the material begins to transform to martensite at a temperature equal toM_(s), the martensitic start temperature, and completes itstransformation to martensite at a temperature equal to M_(f), themartensitic finish temperature.

The shape memory effect of nitinol is demonstrated by shaping thematerial in the relatively high-temperature austenitic phase and settingthe shape by an appropriate heat treatment. Upon cooling the materialbelow the martensitic transformation temperature, the material can bedeformed to a second shape configuration while in the martensitic state.Upon heating to temperatures above the austenitic transformationtemperature the material will return to its original shapeconfiguration. Conventional nitinol materials can typically recover upto 8% strain by this shape memory effect (reference ASM Handbook, Volume2, Shape Memory Alloys, Darel Hodgson et al., page 899).

The superelastic effect of nitinol is demonstrated by the application ofstress to the nitinol material at temperatures above the austenitictransformation temperature, and below M_(d), the maximum temperature atwhich stress-induced martensite can be formed. The initial applicationof stress in this case causes the austenitic structure to deform in theclassical Hookean linear elastic manner until a critical stress isachieved. The application of stress beyond this critical stress resultsin a nonlinear stress-strain response due to the isothermal reversibletransformation to martensite. Upon removal of the applied stress, thematerial can reversibly transform back to austenite, returning to itsoriginal shape. As noted previously, conventional nitinol materials canrecover approximately 6-8% strain by this superelastic effect.

The alternating in-vivo load conditions (due to changes such as betweensystole and diastole) often limit the design of medical devices such asstents and stent-grafts due to the fatigue capability of nitinolmaterials. Improvements in the fatigue performance of nitinol aredesirable to provide an increased fatigue life and fatigue life safetyfactor and to increase design flexibility for implantable medical todevices that include nitinol.

Various publications describe the fatigue resistance of devices madefrom shape memory materials. European Patent Application EP1170393describes a method for improving fatigue performance of actuators madefrom materials that have shape memory effect. The process includesintroducing significant cold work, applying stress in the expectedloading direction, and heating above the recrystallization temperaturefor short times to create a uniform, fine-grained, microstructure.

According to a published article, “Cyclic Properties of SuperelasticNitinol: Design Implications” (SMST-2000: Proceedings of theInternational Conference on Shape Memory and Superelastic Technologies,D. Tolomeo, S. Davidson, and M. Santinoranont, pp. 471-476)strain-controlled fatigue tests were conducted with various pre-strainconditions up to 6% pre-strain. Samples were subjected to strains up to6%, then unloaded to a specified cyclic displacement. The endurancelimits for different pre-strain values remained relatively constant.

A published article titled “Effect of Constraining Temperature on thePostdeployment Parameters of Self-Expanding Nitinol Stents” (SMST-2000:Proceedings of the International Conference on Shape Memory andSuperelastic Technologies, Martynov and Basin, pp. 649-655) describesthe evaluation of retaining temperature on the post deploymentparameters of 28 mm aortic-size stents having a typical diamond shapedstent cell structure. The article states that “The maximum deformationof any stent element in the fully compressed state (when the stent ispacked into a delivery catheter) should not exceed the availablereversible deformation limit, which is about 6 to 8%, depending on thematerial used.”

In another published article, “Fatigue and Fracture Behavior ofNickel-Titanium Shape Memory Alloy Reinforced Aluminum Composites,”authors Porter and Liaw describe an aluminum matrix composite reinforcedwith discontinuous nitinol particulates by powder metallurgy processing.The reinforced composite material is cold rolled at minus thirty degreescentigrade (−30° C.). Upon re-heating, the nitinol transforms toaustenite creating residual internal stresses around each particle tostrengthen the material. Improved fatigue life was observed compared tothe unreinforced control matrix material.

An article entitled “The Study of Nitinol Bending Fatigue” (W. J.Harrison and to Z. C. Lin, SMST-2000, Proceedings of the InternationalConference on Shape Memory and Superelastic Technologies) describesfatigue testing of nitinol samples subjected to alternating strain tosimulate the effects of changing strain resulting from systole anddiastole, and optionally subjected to an additional constant strain(mean strain) that would be expected to result from the interferencebetween an expanded stent and the vessel into which it has been fitted.The samples tested were cut from nitinol tubing. The samples showed goodfatigue life, with the fatigue life being greater for samples exposed tohigher mean strain. This result suggests that that the samples hadapparently been cut at their small diameter (i.e., the “compacted”diameter appropriate for insertion of such a device into a body conduit)and subsequently expanded to a larger diameter at which they weretested, as opposed to having been cut at the larger, expanded diameterand then compressed slightly to create the mean strain.

SUMMARY OF THE INVENTION

The present invention relates to articles made of shape memory alloyshaving improved fatigue performance and to methods of treating articlesformed from shape memory alloy materials by pre-straining the articles(or desired portions of the articles) in a controlled manner so that theresultant articles exhibit improved fatigue performance.

The shape memory articles are preferably medical devices, morepreferably implantable medical devices. They are most preferably devicesof nitinol shape memory alloy, most particularly that is superelastic atnormal body temperature (approximately 37° C.).

Implantable medical devices are those devices that are intended toremain within a living body for periods of 24 hours or longer.

The shape memory alloy articles may be produced from materials ofvarious shapes, such as wire of various transverse cross sectionalshapes including circular, elliptical, square, rectangular, etc.Alternatively, the articles may be made by machining precursor formssuch as sheets, tubes or rods, as by electrical discharge machining(EDM), laser cutting, chemical milling, or the like.

The pre-straining method of the present invention as performed on sucharticles includes the controlled introduction of non-recoverable tensilestrains greater than about 0.20% at the surface of a desired portion ofa shape memory alloy article. Controlled pre-straining operations of theshape-set nitinol metal are performed to achieve non-recoverable tensilestrain greater than about 0.20% at or near the surface of selectedregions in the nitinol metal article. The pre-straining operationsresult in a significant increase in fatigue life of the selectivelytreated regions and an overall improvement in the fatigue performance ofthe device. The pre-straining treatments described in this invention areuseful for increasing the fatigue life safety factor of currentnitinol-based medical devices and for incorporating into the design offuture implantable medical devices that include nitinol, therebyproviding additional design flexibility.

Controlling the amount of pre-strain involves pre-straining the shapememory metal by the controlled application of bending, torsional or acombination of these and/or other forces at pre-determined temperatures.These amounts of pre-strain (resulting in at least about 0.20%non-recoverable strain) may be calculated by analytical methods such asfinite element analysis or the like, in conjunction with the material'sloading and unloading behavior.

Non-recoverable tensile strain is intended to mean the permanent set,i.e., the plastic deformation that remains upon releasing the tensilepre-strain or stress, arising from the displacement of atoms to newlattice sites, as determined by representative material stress-strain(loading and unloading) behavioral properties, or as measured bytechniques such as microhardness testing, x-ray diffraction, backscatterelectron Kikuchi patterns, synchrotron radiation, convergent beamelectron diffraction or the like.

The method of this invention involves pre-straining articles such thattargeted surface regions are subjected to tensile pre-strains exceedingthe recoverable strain limit of the material (typically 6%-8% strain),while maintaining a significant portion of the subsurface area (lessaffected by the pre-strain) within the superelastic material limit.Tensile pre-strains of this type may be induced by the application offorces such as bending or torsional forces. Upon removing thepre-straining force, the lesser-affected superelastic subsurface regionof the article allows the bulk article to recover a significant level ofstrain, such that the article, following the removal of thepre-straining force, returns to or near to its original geometry.

This process thus results in desired local surface regions of thepre-strained article being in a state of compression. A residualcompressive stress state has thus been induced at the targeted surfaceregion. The result is a significant improvement in fatigue performanceof targeted regions of the article subjected to this pre-strainingoperation due to the introduction of residual compressive surfacestresses.

The process of inducing compressive residual surface stresses at desiredlocations by the controlled pre-straining operation of the presentinvention, may also produce a concomitant surface region which issubjected to compression, on the side of the article opposite thetargeted region subjected to tension during the pre-straining operation.The compressive strains introduced on the regions opposite the targetedregions may also exceed the recoverable strain limit of the material,resulting in a residual state of tension at these regions upon removalof the pre-straining load. The end result of the pre-straining operationdisclosed in this invention is the improvement in fatigue performance atthe targeted regions of the medical article, thus resulting in a morefatigue resistant device. This operation can thus be applied tospecifically chosen regions of a medical device where service fatigueloading is most severe and improved fatigue performance is desired, orover the entire surface region of the article.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1A shows a perspective side view of a nitinol alloy wire ofcircular cross-section subjected to a pre-straining operation by acontrolled bending operation.

FIG. 1B shows a transverse cross-sectional view taken through the wireof FIG. 1A indicating representative strain contours for the selectivelytreated cross-sectional area of the wire.

FIG. 1C shows a view of a shape-set nitinol wire specimen: testspecimens are pre-strained following the shape-set heat treatment, whilecontrol specimens are not.

FIG. 2 shows stress-strain curves for nitinol wires subjected topre-straining that results in non-recoverable tensile strain of lessthan about 0.20% and for inventive wires subjected to tensilepre-straining at 37° C. that results in non-recoverable tensile strainof greater than about 0.20%.

FIG. 3 shows stress-strain curves for nitinol wires subjected topre-straining that results in non-recoverable tensile strain of lessthan about 0.20% and for inventive wires that results in non-recoverabletensile strain of greater than about 0.20%, loaded at −30° C. in tensionfollowed by unloading at −30° C. from various pre-strain levels, andheated in the stress-free state to 37° C.

FIG. 4 shows stress-strain curves for nitinol wires loaded at −30° C. intension, heated to 37° C. while maintained at various pre-strain levels,followed by unloading at 37° C. from the various pre-strain levels.

FIG. 5 shows a graph of the non-recoverable strain achieved in nitinolwire when subjected to various methods of tensile pre-straining.

FIG. 6 shows stress-strain curves for nitinol wires subjected to tensilepre-straining treatments at various elevated temperatures.

FIG. 7 shows a fitted Weibull fatigue survivability plot for a group ofnitinol wire samples provided with tensile pre-straining treatment inaccordance with embodiments of the present invention, compared to a nonpre-strained control group when both groups were subjected to an axialfatigue test.

FIG. 8 shows a fitted Weibull fatigue survivability plot for a group ofnitinol wire samples provided with bending pre-straining treatment inaccordance with embodiments of the present invention, compared to thenon pre-strained control group when both groups were subjected to aflexural fatigue test.

DETAILED DESCRIPTION OF THE INVENTION

The present invention relates to methods of treating implantable medicaldevice components formed from nitinol materials (such as nitinol wire)so that the resultant device exhibits improved fatigue performance. Thisinvention identifies methods to induce non-recoverable tensile straingreater than about 0.20% strain at or near the surface of selectedregions of the nitinol metal by controlled pre-straining processes. Thenon-recoverable tensile strain may thus be greater than about 0.25%,0.3%, 0.35%, 0.4%, 0.45%, 0.5%, 0.6%, 0.7%, 0.8%, 1.0%, 1.25%, 1.5%, and2.0%.

Implantable medical devices are typically designed such that the maximumdeformation of any portion of the nitinol material does not exceed theavailable reversible deformation limit, typically 6% to 8% strain, afterthe shape setting treatments (“Effect of Constraining Temperature on thePostdeployment Parameters of Self-Expanding Nitinol Stents,” SMST-2000:Proceedings of the International Conference on Shape Memory andSuperelastic Technologies, Martynov and Basin). The purpose ofmaintaining maximum principle strains below the reversible deformationlimit after the shape setting process has been completed is to ensurethat the device will preserve its original shape.

The reversible deformation limit is defined as the maximum strain amaterial can undergo without inducing non-recoverable strain (i.e.,permanent set) greater than about 0.20%.

The methods of this invention involve the controlled pre-straining ofdesired portions of nitinol articles such that targeted surface regionsare subjected to tensile strains exceeding the about 0.20% recoverablestrain limit of the material, while maintaining a significant portion ofthe cross-section within the superelastic material limit. Upon removingthe pre-straining force, the superelastic region of the structure allowsthe bulk structure to recover significant levels of strain such that thestructure returns to, or near to, its original geometry. This processthus results in desired regions of the pre-strained material that hadbeen subjected to tensile strains beyond their recoverable limit to bein a state of compression. A residual compressive stress state has thusbeen induced at the targeted surface regions. This process results in asignificant improvement in fatigue performance of targeted regions ofthe structure subjected to this pre-straining operation due to theintroduction of residual compressive surface stresses. The controlledprocess of pre-straining can be accomplished by flexural loading,torsional loading, or any combination of loading conditions designed toinduce non-recoverable tensile strains greater than about 0.20% at ornear the surface of fatigue-critical locations of a shape-set, nitinolcontaining implantable medical device.

An example of a pre-straining method included as an embodiment of thisinvention is illustrated in FIGS. 1A and 1B. FIG. 1A shows a nitinolwire of circular cross-section being deformed by a controlled bendingpre-straining operation, indicated by the arrows. The transversecross-section of FIG. 1B shows constant strain contours of the indicatedcross-section of the wire during the pre-straining, bending operation inaccordance with FIG. 1A, with the region showing arcuate iso-straincontours near location A representing the region subjected tonon-recoverable tensile strains greater than about 0.20%. In FIG. 1B,the region showing the iso-strain contours near location B is subjectedto compressive strains. Upon removal of the pre-straining force, thesuperelastic nature of the bulk material (i.e., generally the materialoutside of the regions indicated by the iso-strain contours) forces thematerial to return to, or near to, its original shape. This operationthus induces residual compressive stresses at or near the surfacelocation A and residual tensile stresses at or near the surface locationB. The result of this pre-straining operation is an improvement in thefatigue performance of the treated region A. This pre-straining methodmay thus be applied in a controlled manner to treat desiredfatigue-critical locations of a device.

FIG. 1C shows a view of a shape-set nitinol wire specimen. Wire specimen10 is formed around pins 11, 12, and 13 and includes about 1.25 turns ofthe wire around each of pins 11 and 13 as shown, to create loops 14. Thetwo opposing loops (14) are wound in opposite directions (i.e.,clockwise and counter-clockwise). The distance between pins 11 and 12 isdefined as dimension “A” with dimension “B” (partially defining thelocation of pin 12, parallel to dimension “A”) being half of dimension“A”. Dimension “C” finally defines the location of pin 12 as thedistance pin 12 is located above a line between pins 11 and 13. Pins 11and 13 are of equal diameter. The diameter of pin 12 is chosen toprovide the desired radius at the apex 15 of the wire specimen. Afterbeing formed as shown, these wire specimens 10 are subjected toshape-set heat treatment prior to fatigue testing. Test specimens arepre-strained following the shape-set heat treatment, while controlspecimens are not.

FIG. 2 shows stress-strain curves for nitinol wire samples loaded at 37°C. in tension, followed by unloading at 37° C., from various pre-strainlevels. It is noted that the particular stress-strain response isdependent upon such factors as alloy composition and thermal andmechanical process histories. The resultant non-recoverable tensilestrain (i.e., permanent set) increases with increasing pre-strain level.Information obtained from this type of family of stress-strain curves,in conjunction with analytical procedures such as finite elementanalysis, can be utilized to develop an appropriate temperature,controlled pre-straining (bending) process. This process is designed toinduce non-recoverable tensile strain levels greater than about 0.20% atdesired, fatigue-critical surface locations of a shape-set nitinolstructure. It is apparent this process can be developed for othertemperatures as well.

FIG. 3 shows a representative family of stress-strain curves for nitinolwire samples which have been loaded at −30° C. in tension, followed byunloading at −30° C. from various pre-strain levels, and heated in thestress-free state to 37° C. The resultant non-recoverable tensile strainincreases with increasing pre-strain level. This family of stress-straincurves, in conjunction with analytical procedures such as finite elementanalysis, can similarly be utilized to develop an appropriatetemperature, controlled pre-straining process designed to inducenon-recoverable tensile strain levels greater than about 0.20% atfatigue-critical surface locations of a shape-set nitinol structure.

FIG. 4 shows another representative family of stress-strain curves fornitinol wire samples loaded to various pre-strain levels at −30° C. intension, heated to 37° C. while maintained at their respectivepre-strain condition, followed by unloading at 37° C. from theirrespective pre-strain condition. The resultant non-recoverable tensilestrain increases with increasing pre-strain level. This family ofstress-strain curves, in conjunction with analytical procedures such asfinite element analysis, can similarly be utilized to develop anappropriate temperature, controlled pre-straining process designed toinduce non-recoverable tensile strain levels greater than about 0.20% atfatigue-critical surface locations of a shape-set nitinol structure.

FIG. 5 shows a plot of non-recoverable tensile strain as a function oftensile pre-strain level for the various controlled pre-strainingprocedures described in FIGS. 2-4. Curve A describes samples subjectedto tensile pre-strain at 37° C. and then unloaded at 37° C. (as shown inFIG. 2). Curve B describes samples subjected to tensile pre-strain at−30° C., unloaded at −30° C. and then heated to 37° C. (as shown in FIG.3). Curve C describes samples subjected to pre-strain at −30° C. andthen heated to 37° C. in the pre-strained condition, and subsequentlyunloaded at 37° C. (as shown in FIG. 4). This type of plot, inconjunction with analytical procedures such as finite element analysis,can be utilized to develop an appropriate temperature, controlledpre-straining process designed to induce non-recoverable tensile strainlevels greater than about 0.20% at fatigue-critical locations of ashape-set nitinol structure.

Additional techniques may be utilized to decrease the recoverable strainlimit for given pre-strain levels to allow for the introduction of thedesired non-recoverable tensile strain at the fatigue-criticallocations. These techniques can be incorporated to allow for theintroduction of non-recoverable tensile strains at relatively lowpre-strain levels (less than 6% to 8% pre-strain). Such techniquesinclude but are not limited to, chemical compositional alloymodifications, thermal and mechanical process history modifications,surface modification techniques such as laser surface treatments, or thelike.

FIG. 6 shows a family of stress-strain curves for nitinol wire samplesloaded in tension to 6% pre-strain and unloaded at various temperatures.The resultant non-recoverable tensile strain is shown to increase withincreasing temperature. This provides an example of one technique, byusing an elevated temperature pre-straining, which can be used to createrelatively high (greater than about 0.20%) non-recoverable strains atrelatively low pre-strain levels. This type of plot can be used, inconjunction with analytical procedures such as finite element analysis,to develop an appropriate temperature, controlled pre-straining processdesigned to induce non-recoverable tensile strain levels greater thanabout 0.20% at fatigue-critical locations of a shape-set nitinolstructure. The use of elevated temperature, controlled pre-strainingprocesses can be utilized to induce non-recoverable tensile strainlevels greater than about 0.20% at fatigue-critical locations of ashape-set nitinol structure.

Another technique to provide for the induction of significant (greaterthan about 0.20%) non-recoverable tensile strains at relatively lowpre-strain levels includes the use of a composite structure consistingof a superelastic nitinol core material and an outer surface materialwith limited recoverable strain capability. The outer material mayinclude a nitinol material with an A_(f) greater than 37° C., preferablya nitinol material with an A_(s) greater than 37° C. Alternatively, theouter surface material may also be stainless steel, or any othermaterial with a lower recoverable strain limit than the nitinol corematerial. The use of such a composite material can allow theintroduction of significant non-recoverable tensile surface strains atrelatively low pre-strain levels. The induction of non-recoverabletensile surface strains greater than about 0.20% at of near the surfacemay be introduced by pre-straining the material by bendingpre-straining, torsional pre-straining, or a combination of complexpre-strain loading conditions.

The process of inducing compressive residual surface stresses by thepre-straining operations described herein may also produce a concomitantsurface region which is subjected to compressive strains, occurring onthe opposite surface region of the targeted region subjected to tension,during the pre-straining operation. The compressive strains introducedon the regions opposite the targeted regions may also exceed therecoverable strain limit of the material, resulting in an undesirableresidual state of tension at these regions which may result in reducedfatigue life.

FIG. 1C shows the test specimen in a relaxed condition, wherein apex 15contains no significant residual stresses. Following controlledpre-straining caused by moving pins 11 and 13 closer together, the outerradius of apex 15 of the specimen as shown in FIG. 1C will be in thestate of residual compression stress while the inner radius will be inthe state of residual tensile stress. This method of pre-straining isthus desired when the critical fatigue location is the outer radius ofapex 15. Alternatively, if the fatigue-critical location is the innerradius of apex 15, pre-straining is accomplished by moving pins 11 and13 further apart. In service, the fatigue-critical location is one thathas been previously pre-strained in tension, thus inducing residualcompressive stress at that fatigue-critical location.

The end result of the pre-straining operation disclosed in thisinvention is the improvement in fatigue performance at targeted regionsof the medical device structure, thus resulting in a more fatigueresistant device. This operation can thus be applied to specific medicaldevice structure regions where service fatigue loading is most severeand improved fatigue performance is desired, or over the entire surfaceregion of the structure.

In another aspect of the present invention, it is noted that it is notuncommon for nitinol articles including implantable medical articles tobe subjected to surface modification by various methods such aselectropolishing and shot peening. These methods are known to reduce anynon-recoverable strain at the surface of these articles. Consequently,it is appropriate that any desired surface modification is performedprior to the controlled pre-straining operations as taught by the methodof the present invention.

Example 1

Axial fatigue tests were conducted using superelastic nitinol wiresamples subjected to different tensile pre-strain conditions. Thenitinol wire (Fort Wayne Metals, Fort Wayne, Ind., nominal diameter0.305 mm) utilized for these tests was electropolished to a diameter of0.300 mm and heat treated in air to obtain a straight configuration andto impart superelastic behavior at 37° C. (A_(f)<37° C.) with apermanent set of less than 0.20% when loaded to 6% strain and unloadedat 37° C.

An Instron servohydraulic test machine (Canton, Mass., model no. 8841)was used for the axial fatigue testing. The testing was performed in anair thermal chamber set at 37° C. (+/−1° C.). Wavemaker software (FastTrack 2, Wavemaker Editor/Runtime, version 7.0.0, provided by Instron)was used to generate and execute the axial fatigue tests using adisplacement controlled sine waveform. Test specimen gauge length was100 mm, held with flat-faced grips (Instron PN 2716-016). Five specimenswere pulled to 104 mm length (4% mean strain), and cycled ±0.5 mm (0.5%alternating strain) at cyclic frequencies until failure by fracture, asshown in Table 1. Three additional specimens were pulled to 108 mmlength (8% pre-strain), released to 104 mm length (4% mean strain), andcycled ±0.5 mm (0.5% alternating strain), at a cyclic frequency of 12 Hzuntil fracture (Table 2). An additional three specimens were pulled to106 mm length (6% pre-strain), released to 104 mm length (4% meanstrain), and cycled ±0.5 mm (0.5% alternating strain), at a cyclicfrequency of 12 Hz until fracture (Table 3).

Test results as presented in Tables 1-3 show an increase in fatigue lifewith an increase in pre-strain level.

TABLE 1 No pre-strain, 4% mean strain, 0.5% alternating strain cyclicSpecimen frequency CTF 1  8 Hz 3,852 2  8 Hz 2,998 3 15 Hz 3,383 4 12 Hz3,868 5 12 Hz 3,988 mean CTF: 3618 cycles

TABLE 2 8% pre-strain, 4% mean strain, 0.5% alternating strain cyclicSpecimen frequency CTF 6 12 Hz 9,266 7 12 Hz 9,779 8 12 Hz 9,533 meanCTF: 9526 cycles

TABLE 3 6% pre-strain, 4% mean strain, 0.5% alternating strain cyclicSpecimen frequency CTF 9 12 Hz 6,185 10 12 Hz 7,520 11 12 Hz 7,541 meanCTF: 7082 cycles

The axial fatigue test results are summarized in FIG. 7, showing afitted Weibull distribution fatigue survival plot comparing the fatiguelives for different groups of nitinol wire samples (plotted asproportion of survivors within each group versus number of cycles tofailure, or CTF).

Example 2

Flexural fatigue tests were conducted using superelastic nitinol wire(Fort Wayne Metals, Fort Wayne, Ind., nominal diameter 0.323 mm) samplessubjected to different tensile pre-strain conditions. The nitinol wireused for these tests was electropolished to a diameter of 0.321 mm.

Thirty wire test specimens were formed into the shape described in FIG.1C, by winding the wire around the 0.79 mm diameter stainless steel pins11, 12 and 13 of the heat treatment fixture, as shown in FIG. 1C. Alltest specimens were heat treated in air while on the fixture to set thetest sample geometry configuration and to impart superelastic behaviorat 37° C. (A_(f)<37° C.) with a permanent set of less than 0.20% whenloaded to 6% strain and unloaded at 37° C. Dimension “A” between pins 11and 13 (center-to-center) was 13.72 mm, while dimension “B” was half ofdimension “A”. Dimension “C” was 5.08 mm. The support loops 14 at theends of each sample 10 were of an inside diameter that conforms to thediameter of pins 11 and 13. The apex 15 of each test specimen 10 wasformed to a radius (at the inside radius of the apex bend) thatconformed to the diameter of pin 12.

Prepared test specimens were divided into three separate groups (10samples per group): a control group (Group 1: no pre-strain), a roomtemperature pre-strain group (Group 2), and a cold pre-strain group(Group 3). Each sample from Group 2 was pre-strained by placing thesample eyelet support loops (14) onto the same pin (11) to pre-strainthe test sample apex (15) at room temperature. The test specimens werekept at this condition for 2 hours at room temperature and then removed.Group 3 samples were placed into a bath mixture of dry ice and 100%isopropyl alcohol, with a submersed thermocouple to monitor bathtemperature. The samples were then pre-strained while submersed in thebath, following the same pre-strain procedure described for Group 2. Thebath temperature ranged from −34° C. to −14° C. during the pre-strainingprocedure. The samples were removed from the bath while in theconstrained condition and placed in room temperature air for 2 hoursprior to removal of the pre-strain constraint. Group 1 was notpre-strained, and served as control samples for the subsequent fatiguetests. The maximum principle tensile pre-strain level at thefatigue-critical location (outside radius surface of the apex) wascalculated to be approximately 8.5%. This maximum principle pre-strainlevel was calculated by applying standard engineering mechanics formulas(straight and curved beam deflection equations, from “Roark's Formulasfor Stress & Strain,” 6^(th) edition, McGraw Hill, New York, N.Y.) tothe specimen geometry.

Fatigue tests were conducted using a fatigue tester designed and builtfor the purpose of conducting cyclic, deflection controlled, fatiguetesting of apical wire samples of the previously described geometry. Thetester is designed to accommodate up to forty test samples. Wire fatiguetest samples were loaded onto the fatigue test apparatus by placing thetest sample support loops onto 0.79 mm diameter stainless steel pins ofthe fatigue tester. The tester was set to alternate test pin deflectionsfrom 9.20 mm and 10.16 mm (i.e., dimension “A” of FIG. 1C alternatedbetween 9.20 mm and 10.16 mm). These deflections were selected toachieve a maximum principle mean tensile strain of 2.5% and analternating strain of 0.3% at the outside radius of the test specimenapex. The maximum principle strains for these deflections was calculatedby applying standard engineering mechanics formulas (straight and curvedbeam deflection equations, from “Roark's Formulas for Stress & Strain,”6^(th) edition, McGraw Hill, New York, N.Y.) to the specimen geometry.

These deflections were set-up using a telescoping dial depth gage andgage blocks. All 30 specimens were mounted on the tester, with test andcontrol samples being placed alternately along the test fixture. Thefatigue testing was performed in a 37±1° C. water bath and at a cyclicfrequency of approximately 18 Hz.

The flexural fatigue test results are summarized in FIG. 8, showing afitted Weibull distribution fatigue survival plot comparing the fatiguelives for different groups of nitinol wire samples (plotted asproportion of survivors within each group versus number of cycles tofailure by fracture at the apex, or CTF). Data are presented in Tables4-6 for Groups 1-3 respectively. Various specimens from Groups 2 and 3survived the 40 million cycle length of the tests as noted. The fatiguetest results demonstrate an improvement of approximately three orders ofmagnitude in the mean fatigue lives for the pre-strained sample groups.This example demonstrates the significant improvement in the fatigueperformance of nitinol (particularly nitinol wire) when subjected to apre-straining treatment.

TABLE 4 Controls, No Pre-Strain Specimen CTF 1 13,002 2 17,004 3 20,0004 20,000 5 23,006 6 24,002 7 24,002 8 24,002 9 29.006 10 37,002

TABLE 5 Room Temperature Pre-Strain Specimen CTF 1   125,055 2 1,300,0003 4,148,832 4 4,246,188 5 12,408,376  6 40,000,000+ 7 40,000,000+ 840,000,000+ 9 40,000,000+ 10 40,000,000+

TABLE 8 Cold Pre-Strain Specimen CTF 1   755,022 2 2,229,536 3 2,399.9994 2,481,166 5 2,817,037 6 7,723,746 7 8,242,257 8 9,278,477 940,000,000+ 10 40,000,000+

While the principles of the invention have been made clear in theillustrative embodiments set forth herein, it will be obvious to thoseskilled in the art to make various modifications to the structure,arrangement, proportion, elements, materials and components used in thepractice of the invention. To the extent that these variousmodifications do not depart from the spirit and scope of the appendedclaims, they are intended to be encompassed therein.

1. A self-expanding stent comprising a frame formed from nitinolexhibiting superelastic behavior at body temperature, the frame beingconfigured to self-expand from a constrained delivery profile to anexpanded operative profile, wherein the frame is pre-strained by theapplication of a pre-straining torsional force to an extent thatselectively induces at least about 8.0% tensile strain at or near only asurface of at least one selected location on the frame duringcircumferential compaction of the frame to a smaller size, whereinfollowing release of the pre-straining force, the frame returns to, ornear to, its geometry prior to the application of the pre-strainingforce.
 2. A self-expanding stent according to claim 1 wherein thepre-straining results in at least about 8.5% tensile strain.
 3. Aself-expanding stent according to claim 1 wherein the stent is astent-graft.
 4. A self-expanding stent according to claim 2 wherein thestent is a stent-graft.
 5. A self-expanding filter device comprising aframe formed from nitinol exhibiting superelastic behavior at bodytemperature, the frame being configured to self-expand from aconstrained delivery profile to an expanded operative profile, whereinthe frame is pre-strained by the application of a pre-strainingtorsional force to an extent that selectively induces at least about8.0% tensile strain at or near only a surface of at least one selectedlocation on the frame during circumferential compaction of the frame toa smaller size, wherein following release of the pre-straining force,the frame returns to, or near to, its geometry prior to the applicationof the pre-straining force.
 6. A self-expanding filter device accordingto claim 5 wherein the pre-straining results in at least about 8.5%tensile strain.